Re: Assy question

From: Marko Mäkelä (
Date: 2001-03-15 09:03:02

On Thu, 15 Mar 2001, Ruud Baltissen wrote:

> The only thing I can think of is that the author made the same mistake as I
> did: assuming that the above 1 byte commands only take one cycle. Personal
> remark: why do they need two???

Because the processor decodes the instruction opcode while fetching the
second byte of the instruction.  In the beginning of the 2nd cycle, it
does not know whether it'll be a one-byte instruction or not.  This
greatly simplifies the design, I think.

Another peculiarity is the JSR instruction.  The high byte of the address
is not read until the last (6th) cycle.  If I remember correctly, also the
return address pushed on the stack is incremented by one by the RTS


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